Abstract: The fracture toughness of a nearly lamellar Ti-45Al-8Nb-0.2W-0.2B-0.1Y alloy and a fully lamellar Ti-45Al-7Nb-0.2W-0.2Hf-0.3B-0.15 C alloy at 750℃ was investigated by prefabricating a fatigue crack in the compact tension(CT) specimen,and the corresponding fracture morphologies were observed by optical microscopy and scanning electron microscopy. It is found that the fracture toughness of the nearly lamellar high Nb-containing Ti Al alloy is 19.54 MPa·m^1/2 at 750℃,obviously lower than the value of 31.58 MPa·m^1/2 for the fully lamellar alloy. The maximum cyclic load at which the fatigue crack initiates in the nearly lamellar alloy is noticeably less than that in the fully lamellar alloy. Fracture images show that for the nearly lamellar alloy,cracks initiate mainly in equiaxed γ grains and propagate in different ways,intergranularly or transgranularly in γ grains or lamellar colonies. But for the fully lamellar alloy,cracks initiate mainly at lamellar interfaces perpendicular to the load direction and propagate in both interlamellar and translamellar ways,in accompanying with secondary crack initiation.